<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .container{
width: 100px;
height: 100px;
background: red;
animation:  run 1s reverse infinite;/*1秒内，无限循环(infinite)以直线方式把宽度从100到800 */
        }
       @keyframes run {  
            0%{
                width: 100px;
            }
            100%{
                width: 800px;
            }
        }
    </style>
</head>
<body>
    <div class="container"></div>
</body>
</html>